Abstract
Recently, luminescent nanomaterials with one-dimensional (1D) structured form have been extensively investigated as fluorescent label agents in biology. Among them, 1D nanostructured terbium compounds (for example terbium phosphate) have attracted great attention due to their high-emission efficiency in water. In this report, terbium phosphate nanorods were successfully prepared by hydrothermal method in autoclave for the first time. This method is a simple one, which permits obtaining large-scale, uniform and pure product. The size of the rods can be controlled precisely. The nanorods have diameters from 15 to 50 nm, and lengths from 300 to 500 nm. Based on the effects of change of pH values to the size, crystalline structure and morphology of terbium phosphate nanorods, field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD) data of the as-synthesized samples have been elucidated. The photoluminescent (PL) spectra of TbPO4 nanorods have been measured at room temperature under ultraviolet excitation and show four high luminescence main peaks at 490, 545, 585 and 620 nm.
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1. Introduction
One-dimensional (1D) nanomaterials have attracted much attention over the past decade due to their unique properties and potential applications in nanodevices [1–3]. All materials having at least one dimension between 1 and 100 nm are called nanostructured materials, and have received steadily growing interest because of their peculiar and fascinating properties, and applications superior to their bulk counterparts. Many attempts have been made to fabricate 1D nanomaterials using template techniques so far [4–6]. Recently, rod, tube and wire nanostructures have stimulated intensive world-wide research activities because their morphology is associated with an intrinsic multifunction arising from different contact regions. These properties make them also very promising candidates for highly functional and more effective devices [7–9].
Rare earth oxides are a kind of advanced material and have been widely used as high-performance luminescent devices, magnets, catalysts and as novel functional units in nanophotonics. Recently, inorganic fluorescent nanorods have attracted scientists in biology and medicine [5,10]. According to some reports, there is drastic reduction of the plasmon dephasing rate in nanorods compared to small nanospheres due to a suppression of interband damping. These rods show very little radiation damping due to their small volumes. Therefore, we are highly interested in fabrication of nanorods. To date some rare earth oxide nanorods have been successfully fabricated by various chemical methods. Based on studying the different reported chemical nanosyntheses we designed a soft template-assisted hydrothermal route to obtain Tb(OH) 3 nanorod [11].
In this report terbium phosphate nanorods were successfully prepared by hydrothermal method in the autoclave for the first time. This method is simple and permits obtaining large-scale, uniform and pure product. The size of the rods can be controlled precisely with diameters from 10 to 50 nm and lengths from 300 to 500 nm. The morphology and the crystal structure of the as-synthesized samples have been investigated by thermal analysis and observed using field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The photoluminescent (PL) spectra of TbPO 4 rods have been measured at room temperature under ultraviolet (UV) excitation and show four main peaks at 490, 545, 585 and 620 nm.
2. Experimental
2.1. Synthesis
Terbium phosphate (TbPO 4) nanorods were prepared by the hydrothermal route from TbCl 3·H 2 O (Sigma-Aldrich, 99.9%), NH 4 H 2 PO 4 (Merck, 99%) in a round-bottomed flask and this was followed by magnetic stirring for 120 min. The pH of the solution after the reaction was in the range of 1–10 by adding NaOH 1M. After that, the solution was heated at 200 °C for 20 h, and then cooled down slowly to room temperature. The resulting products were collected and centrifuged at 5900 rpm. The precipitate was washed several times using water and then dried in air at 60 °C for 6 h. The chemical reaction is shown as follows:
2.2. Characterization
The morphology and structure of the as-synthesized samples was observed by using FE-SEM (Hitachi, S-4800), transmission electron microscopy TEM (JEM-1010) and XRD measurements (Siemens D5000 with λ=1.5406 Å in the range of 15°⩽ 2θ⩽ 75°). The PL spectra of the nanorods were also determined by using a spectrometer Horiba Jobin Yvon IHR 550 under excitation at 370 nm from an LED.
3. Results and discussion
3.1. Morphology
The morphology of the as-synthesized samples was observed with FE-SEM. Figure 1 shows the typical FE-SEM images of the TbPO 4 sample with pH=1, 2, 6, 10 at 200 °C in 20 h. The FE-SEM image of the TbPO4 prepared under conditions pH=1, 200 °C and 20 h indicates that the nanorods have bundle shape with the lengths of rod about 300–500 nm and diameter about 40–50 nm (figure 1(a)). Careful observation of the FE-SEM image indicates that these 1D nanorod bundles are composed of individual parallel nanorods. When increasing to pH=2, the size of rod decreased: the length of rod is about 300–500 nm, and the diameter of TbPO 4 (pH=2, 200 °C and 20 h) nanorods is about 20–30 nm (figure 1(b)).
Figure 1 FE-SEM images of TbPO 4 at pH=1 (a), pH=2 (b), pH=6 (c) and pH=10 (d).
The FE-SEM images of the TbPO 4 (pH=6, 200 °C and 20 h) (figure 1(c)) and TbPO 4 (pH=10, 200 °C, and 20 h) (figure 1(d)), respectively, have nanorod bundle shape with the lengths of rod about 200–300 nm and diameter of rod about 15–20 nm. The FE-SEM image analysis results indicate that when the pH value increased, the size of nanorods decreased. Therefore, the size of the rods can be controlled well by solution pH values without the presence of any other organic additives.
3.2. Phase and structure
Under the identical synthetic conditions, the crystal structure of the TbPO 4 products have been identified by XRD analysis. Figure 2 shows XRD pattern of the TbPO 4 nanorods at pH=2. All diffraction peaks agree well with a hexagonal structure of TbPO 4·H 2 O (PDF card no 20-1244). The XRD patterns of TbPO 4 products with different pH values (pH=1, 2, 6 and 10) are characteristic of a pure hexagonal phase of terbium phosphate hydrate (data not shown).
Figure 2 XRD pattern of the TbPO 4 nanorods at pH=2.
3.3. Photoluminescence
A photoluminescence excitation (PLE) spectrum of the TbPO 4 nanorods is presented in figure 3. The nanorods can either be optically excited in the UV range, or in the visible, in the terbium states. The excitation wavelengths of the TbPO 4 nanorods were 270, 312, 350, 370, 482 nm. Excitation at any of these wavelengths resulted in similar emission spectra for TbPO 4 nanorods.
Figure 3 Room temperature PLE spectrum monitored at 546 nm emission peak of the TbPO 4 nanorods with pH=2.
A PL spectrum of the TbPO 4 nanorods under an excitation wavelength of 370 nm at room temperature is displayed in figure 4. PL spectra of the TbPO 4 nanorods with different pH values (pH=1, 2, 6 and 10) exhibited the same major peaks, but with different intensities. The main emission peaks for TbPO 4 nanorods were observed at 490, 546 (major), 585 and 620 nm (due to the 5 D 4–7 F 6, 5 D 4–7 F 5, 5 D 4–7 F 4, 5 D 4–7 F 3 transitions of Tb 3+, respectively) under excitation at 370 nm. TbPO 4 nanorods yielded the characteristic green emission of Tb 3+, in which the 5 D 4–7 F 5 transition at 546 nm emission was the most prominent band. Such fluorescence properties of TbPO 4 nanorods have attracted a great deal of attention in biology.
Figure 4 PL spectrum of the TbPO 4 nanorods with pH=2 under an excitation wavelength of 370 nm at room temperature.
4. Conclusion
Nanorods of TbPO 4 have been successfully synthesized by hydrothermal method in autoclave for the first time. The TbPO 4 rods have a lengths ranging from 300 to 500 nm and diameters ranging from 15 to 50 nm. The size of the rods can be controlled well by the solution pH values. The as-obtained TbPO 4 nanorods exhibited a pure hexagonal phase in crystal structure. The PL spectra of the TbPO 4 nanorods displayed four narrow emission bands with the strongest one at 546 nm. This demonstrates that nanorods of the TbPO 4 can find promising applications as fluorescent labels in biology.
Acknowledgments
This work is supported by Vietnam Basic Research Programming for application, project code 2/2/742/2009/HD-DTDL. The authors acknowledge National Key Lab of Electronic Materials and Devices in Institute of Materials Science, Vietnam Academy of Science and Technology for use of research facilities.